feat(audit): Batch 10 — CC conventions (shared line-buffer, cleanError)

The contained consistency consolidations; the sweeping ones are deferred with
their standard documented (they're the audit's own post-1.0 refactors, and
several would be reckless to do unattended).

Landed:
- src/main/lib/lineBuffer.ts (createLineBuffer): one newline line-splitter for
  streamed child-process output, replacing the duplicated buffer loop in
  download.ts and terminal.ts (CC3/CC4/SIMP5). Pure + unit-tested
  (test/lineBuffer.test.ts, 7 cases: partial lines, CRLF, empty lines, flush,
  multi-line chunks). download.ts keeps byte-identical marker parsing and no
  close-flush (yt-dlp template lines are always newline-terminated).
- ytdlp.ts: getYtdlpVersion/updateYtdlp now run stderr through cleanError, the
  same `cleanError(r.stderr) || r.error?.message || ''` idiom already used by
  download/probe/indexer (CC6) — a failed check shows the trailing error line.
- CL6 resolved: the one real redundancy was L15; clearDir/openFile/showInFolder
  are deliberate view-model wrappers (CC13), left as-is.

Deferred with documented standard + rationale (CODE-AUDIT.md): CC1 (breaking
persisted-key rename → migration), CC5+CL4 (security-critical updater
net.request, not live-verifiable here), CC7 (cosmetic arg-order), CC9 (zod dep),
CC10 (electron-store↔jsonStore split is deliberate for DPAPI), CC11 (config.ts
move), CC12 (file-tree reorg), CC13→L93, CC14→L142.

Verified: typecheck (node+web) + 275 tests + eslint + production build green;
touched files prettier-clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/download.ts

@@ -15,6 +15,7 @@ import {
 } from './binaries'
 import { getSettings } from './settings'
 import { execFileAsync } from './lib/exec'
+import { createLineBuffer } from './lib/lineBuffer'
 import { getDownloadArchivePath, getDefaultMediaDir } from './paths'
 import { ensureManagedYtdlp } from './ytdlp'
 import { materializeCookies, hasStoredCookies } from './cookies'
@@ -372,7 +373,6 @@ function wireChildProcess(params: {
   const { wc, opts, rec, cleanup, getTitle } = params
   const child = rec.child
 
-  let stdoutBuf = ''
   let stderrTail = ''
   let filePath: string | undefined
   // Latched once the first download stream reports 'finished'; flags later
@@ -415,28 +415,28 @@ function wireChildProcess(params: {
   }
   bumpWatchdog()
 
+  // Split stdout into lines (shared helper, CC3/CC4) and parse our --print markers.
+  // No flush on close: yt-dlp's progress/path template lines are always
+  // newline-terminated, so a trailing partial is never a real marker.
+  const stdoutLines = createLineBuffer((line) => {
+    if (line.startsWith(PROGRESS_MARKER)) {
+      const p = parseProgress(line.slice(PROGRESS_MARKER.length))
+      if (p) {
+        // SR7: yt-dlp reports a 'finished' status when each download stream
+        // completes. A video+audio download has two streams, so the bar would
+        // otherwise fill 0→100% twice. Latch on the first 'finished' and flag
+        // every later tick as "finishing" so the renderer shows an indeterminate
+        // merge state instead of a visible restart.
+        if (p.status === 'finished') finishing = true
+        send(wc, { type: 'progress', id: opts.id, progress: { ...p, finishing } })
+      }
+    } else if (line.startsWith(FILEPATH_MARKER)) {
+      filePath = line.slice(FILEPATH_MARKER.length).trim()
+    }
+  })
   child.stdout?.on('data', (chunk: Buffer) => {
     bumpWatchdog()
-    stdoutBuf += chunk.toString()
-    let nl: number
-    while ((nl = stdoutBuf.indexOf('\n')) >= 0) {
-      const line = stdoutBuf.slice(0, nl).replace(/\r$/, '')
-      stdoutBuf = stdoutBuf.slice(nl + 1)
-      if (line.startsWith(PROGRESS_MARKER)) {
-        const p = parseProgress(line.slice(PROGRESS_MARKER.length))
-        if (p) {
-          // SR7: yt-dlp reports a 'finished' status when each download stream
-          // completes. A video+audio download has two streams, so the bar would
-          // otherwise fill 0→100% twice. Latch on the first 'finished' and flag
-          // every later tick as "finishing" so the renderer shows an indeterminate
-          // merge state instead of a visible restart.
-          if (p.status === 'finished') finishing = true
-          send(wc, { type: 'progress', id: opts.id, progress: { ...p, finishing } })
-        }
-      } else if (line.startsWith(FILEPATH_MARKER)) {
-        filePath = line.slice(FILEPATH_MARKER.length).trim()
-      }
-    }
+    stdoutLines.push(chunk.toString())
   })
 
   child.stderr?.on('data', (chunk: Buffer) => {